Aerosol and Trace Gases
New particle formation
Atmospheric aerosol particles cover a wide size range from ca. 1 nm (0.001 µm) to 100 µm. The smallest of these particles originate from a process termed new particle formation (NPF) or nucleation, which starts with the formation of small clusters consisting only of a few molecules. Most of the times these clusters evaporate rapidly but depending on the conditions they can also form thermodynamically stable particles that can eventually grow and become seeds for cloud droplets (cloud condensation nuclei, CCN, with a diameter above ca. 50 nm). NPF is therefore an important factor when assessing the climatic impact of aerosol particles due to the aerosol-cloud-climate interaction. Model calculations suggest that around half of the CCN originate from new particle formation on a global scale, whereas the other half is due to primary emissions (mainly from sea salt, desert dust, biomass and fossil fuel burning, plant debris, etc.). The health effects of nanoparticles are a second major topic of active research as the tiny particles can penetrate deep into the human body, which can make them potentially more harmful compared with larger particles. However, atmospheric new particle formation and growth is not fully understood yet despite its importance for climate and human health.
The most important factors influencing the new particle formation rate (number of new particles produced per volume and time) include the presence of suitable trace gases, their concentrations, temperature, and, in the case of ion-induced nucleation, also the ionization rate. Compounds that have been identified to be involved in nucleation are mainly sulfuric acid, ammonia, amines, water vapor, iodine oxides, and highly oxygenated organic compounds. However, in most cases one compound alone cannot efficiently nucleate at atmospheric concentrations. Therefore, mixtures of gases (e.g., the ternary system of sulfuric acid, water vapor and ammonia) are generally required and it is not sufficient to measure just one compound in order to characterize NPF. Another important factor that complicates research on atmospheric nucleation is the fact that the relevant trace gases occur only at tiny concentrations. Peak sulfuric acid mixing ratios are usually only around 0.1 pptv (pptv = parts per trillion by volume, i.e., 0.1 pptv corresponds to 1 molecule in 1013 other air molecules). Yet such a minuscule concentration can have a strong impact regarding the health and climate aspects (see above).
Our group investigates the mechanisms that lead to the formation of new particles. This research is addressed by instruments that are designed to quantitatively measure compounds like sulfuric acid, ammonia, amines, diamines, iodic acid and highly oxygenated molecules with extremely high sensitivity at high time resolution. The main instruments deployed are time of flight mass spectrometers equipped with home-built chemical ionization sources operated at atmospheric pressure. These instruments use nitrate reagent ions mainly for sulfuric acid, iodic acid, cluster and HOM measurement and water clusters as reagent ions for ammonia, amine and diamine measurement. An interface for size-selectively collection and evaporation of nanoparticles has been developed recently that can be connected to one of the chemical ionization mass spectrometers. With this set-up the chemical composition of both the gas phase and the particle phase can be measured. Besides the mass spectrometers and commercial trace gas monitors (SO2, O3, NOx, CO, CO2), we use a suite of particle counters and spectrometers to characterize the aerosol size distribution (between approximately one nanometer and tens of micrometers). Some of these instruments have been used during field campaigns (e.g., at Taunus Observatory, Jungfraujoch, Vielbrunn) but they are also used in laboratory experiments (e.g., with a flow reactor). The main project we are involved in is the CLOUD (Cosmics Leaving OUtdoor Droplets) project at CERN. In CLOUD new particle formation and growth is studied in a laboratory experiment where atmospheric conditions can be replicated in an extremely well characterized and controlled stainless steel chamber (26.1 m3 volume). Measurements with one of our mass spectrometers on the HALO aircraft above the Amazon are planned for 2020 in order to characterize nucleation in the upper tropical troposphere.